Process and product of chrome plating nitrided steel



United States Patent 3,205,153 PROCESS AND PRODUCT OF CHROME PLATIN G NITRIDED STEEL Stanley E. Casper, Portland, Conn., assignor to Chandler Evans Inc., a corporation of Delaware No Drawing. Filed May 21, 1962, Ser. No. 196,473 6 Claims. (Cl. 204-37) This invention pertains to the process and product of chrome plating nitrided steel, and more particularly has reference to electroplating nitrided steel machine parts with chromium, in order to reduce coefiicient of friction between moving parts, while retaining the surface hardness and overall toughness of the nitrided steel.

Although chrome plating of nitrided steel machine parts decreases their kinetic coeflicient of friction, and thus increases their resistance to wear, where such parts are subjected to high machine speeds, their useful life has been relatively short, and there has long been a demand for improvement in this direction. Great difficulty has been encountered in applying firmly adherent, hard electroplate to nitrided steel parts because of certain contaminant films formed on the surfaces of the parts during or after the nitriding process; and much effort on the part of those skilled in the art has been devoted to attempts to solve the problem of chrome plating nitrided steel machine parts so as to improve their useful life, but the results of such efforts have heretofore not been satisfactory, in that they have failed to produce such parts having substantially increased useful life.

I have discovered an adequate solution of this problem by devising an improved method of chrome plating nitrided machine parts, as disclosed hereinbelow, which obtains a far better adherence of the chrome plate to the nitrided steel surface, and produces parts having much greater resistance to Wear.

The improved method of my invention comprises three principal features, viz:

(I) Starting the electroplating process with nitrided ferrous alloy steel, in which the nitriding method has been carefully controlled, so as to produce a case (hardened surface layer) whose properties conform to the following specifications:

(a) Total case depth of 0.12" minimum (with a usual range of 0.18.022).

(b) Surface hardness (Rockwell scale of 15 N 94 minimum, (with a usual range in hardness of 94.5- 95.0.)

(c) White layer (iron nitride) .0006" maximum, (with usual range in thickness of .O003"-.0004).

(d) Hardness of 15 N 92 at .004" depth (with usual range in hardness of 15 N 92.5-93.0).

(II) Electroplating the nitrided parts in accordance with the following sequential steps:

(A) The parts are degreased by treatment with neutral trichloroethylene vapor;

(B) The degreased parts are mechanically cleaned, as by wire brushing, sand blasting or tumbling, and are then blasted with an abrasive (water) slurry. I have found that best results are obtained by slurry blasting with a water spray carrying abrasives (e.g. sand or A particles in the range of 80- 1000 grit size.

(C) The degreased and cleaned parts are water rinsed for 10 seconds and electroplated as follows:

(1) The parts are fixtured in contact with the cathode bus bar of a standard electroplating tank, containing a solution of 32-34 oz./ gal. of commercial plating chromic acid, having a sulfate ratio of one 80., radical to every 95- CrO "ice

(2) The anodes (lead) are adjusted to always have a ratio of twice the surface area of the parts being plated.

(3) The solution temperature is maintained between F. to F., and the current density-temperature ratio never exceeds 1 amp/in. per each 45 F. The usual range will be 1 ampere/in. per each 45 F. to 1 ampere/in. per each 60 F. Thus, for example, at a solution temperature of 135 F. the current density would be 3.0 amperes/in. for a current density-temperature ratio of 1 ampere/in. per each 45 F.

(4) The time of deposition will be governed by the amount of plate desired. Best results show a 12-18 minute operation under the above condition, and give chromium electrodeposits of .00015" to .00025 thick. The properties of said chrome will comply to Military Specification QQ-C-320 and AMS 2406.

(5) After said process is complete, the parts are baked at 375 F. for 3 hours and visually inspected. Any discoloration, blisters, peelings are considered unacceptable, and will be reprocessed as described in paragraph HI below.

(111) Reprocess parts that are rejected as unacceptable,

as a result of the above visual and temperature resistance tests, as follows:

(6) The parts are fixtured as in paragraph II (C) (1) above.

(7) Strip the defective chrome plating from parts by immersing parts in an alkaline bath e.g. NaOH), and reversing the current flow, by making the parts the anode, under a voltage potential of 6 volts direct current. Any commercial alkaline electrolytic cleaner (e.g. NaOH and carbonates) is suitable at a concentration of 8 oz./ gal. of water. A temperature of 200 F. minimum is maintained.

(8) The parts are abrasive (Water) slurry blasted as described in paragraph II (B) above.

The features which distinguish my novel process from the nearest prior art process are shown by the following comparison:

Novel Invention Process Prior Art Process (1) Process is started with nitrided (1) Process is started with parts parts having a case limited to whose case has properties (2) Parts are degreased by neutral trichlorethylene vapor method.

(3) Parts are mechanically cleaned by abrasive (water) slurry blasting. (4) Parts are water rinsed (5) Parts are electroplated as specified in paragraph II (C) above with current-tempera ture ratios as stated in subparagraph II (C)(3) above, resulting in chromium electrodeposits of from .00015" to .00025".

(6) Parts are visually inspected for discoloration and parts showing any discoloration are rejected.

At no time are parts chemically treated before electroplating.

(2) Parts may be degreased by use of unneutral hydrocarbons.

(3) Parts are usually cleaned by reverse current method.

(4) Parts are usually desmutted with hydrochloric acid, and reverse current acid etching in chromic acid.

(5) Parts may be electroplated without regard to control limits specified in paragraph II (C) above, and with no concept of plating control current-temperature ratios, as stated in subparagraph II (C)(3) above, resulting in chromium eleetrodeposits of from .00010" to .0020".

(6) Parts showing discoloration on visual inspection are usually accepted.

Parts may be chemically treated (e.g. with acids) before electroplating.

Prior art processes, as indicated, may cause the following rejectable defects:

(1) Peeling and blistering.

(2) Thieving (mainly because the proper current-temperature ratios were not adhered to and the passivity).

(3) Discolorations.

(4) Spots of soft or burned chrome, because of the necessity to use excessive currents in the plating process.

The features which distinguish the product resulting from my novel process, as compared with product of the nearest prior art process are as follows:

(1) The core (ferrous alloy steel) This will be a fine mixture of ferrite and cementite which lends to good shock properties and toughness.

(2) The case (iron nitride in a matrix of ferrous alloy steel)minimum depth .012 (usually .018".022)

This is responsible for increasing fatigue properties and is the necessary hard mass to support the thin-hard surface layers.

(3) White layer or iron nitride This area is extremely hard and is an excellent support for the thin hard-chrome layer. This layer is usually .0003".0004" thick and never exceeds .0006 as brittleness will be a hazard.

(If the chrome were not applied, this coating would increase the surface coefficient of friction to a level where galling and seizing would be a severe problem.)

(4) Chromium layer This layer has a thickness of from .00015" to .00025" and a hardness approaching that of the iron nitride, and has an extremely low coefiicient of friction, ranging from .09 to .12, as compared with a coeflicient ranging from .19 to .30 for uncoated nitrided steel parts. It is this final layer that increases the wear resistance of the plated parts from 50 percent to 100 percent and thus increases the useful life of these parts from one and a half to twice the life of prior art treated parts.

I have found that the very substantial reduction in the coefiicient of friction, and consequent corresponding increase in wear resistance and useful life of nitrided steel parts, which have been chrome-plated in accordance with my invention, is due in large measure to careful control of each step of the process, so that the results obtained by each step always fall well within the range of permissible limits that I have disclosed-hereinabove. These limits are therefore in the nature of critical values that are essential to the successful carrying out of my invention.

While I have described the preferred embodiment of my invention, I do not limit the invention to the precise details disclosed by way of illustration, except to the extent of the permissible range of variations indicated in the several steps of the process, since such details may be altered and varied by those skilled in the art, without departing from the spirit of my invention or exceeding the scope of the appended claims.

I claim:

1. A process for electrolytically chrome plating nitrided ferrous alloy steel machine parts characterized by the following sequential steps: the parts are degreased with neutral trichlorethylene vapor; and then mechanically cleaned by blasting with an abrasive-water slurry; the parts are then water rinsed and fixtured in contact with the cathode bus bar of a standard electroplating tank, and immersed in a solution of commercial plating chromic acid which is maintained at a temperature between F. and F.; the anodes of the tank are adjusted to always have a ratio of twice the surface area of said parts, and electric current is passed for 12-18 minutes through the solution, at a rate such that the currentdensity-temperature ratio ranges from 1 ampere/inch per each 45 F. to 1 ampere/inch per each 60 F., resulting in a chromium electrodeposit of .00015" to @00025" thick; the parts are then baked for 3 hours at maximum depth.

4. Nitrided ferrous alloy steel machine parts electrolytically chome plated as in claim 1.

5. Nitrided ferrous alloy steel machine parts electroyltically chrome plated as in claim 2.

6. Nitrided ferrous alloy steel machine parts electroyltically chrome plated as in claim 3.

References Cited by the Examiner UNITED STATES PATENTS 1,838,273 12/31 McBride 204-37 2,450,296 9/48 Passalacqua 20437 2,800,436 7/57 Stareck 204--37 OTHER REFERENCES Graham: Electroplating Engineering, Reichold Publishing Co., N.Y., 1955.

Morisset et a1.: Chromium Plating, Robert Draper Ltd., Middlesex, England, 1954, pp. 79, 83, 94, 95, 97, 391-394. Sully: Chromium, Butterworths Scientific Publications, London, 1954, pp. 215, 216.

JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, MURRAY TILLMAN, WINSTON A. DOUGLAS, Examiners. 

1. A PROCESS FOR ELECTROLYTICALLY CHROME PLATING NITRIDED FERROUS ALLOY STEEL MACHINE PARTS CHARACTERIZED BY THE FOLLOWING SEQUENTIAL STEPS: THE PARTS ARE DEGREASED WITH NEUTRAL TRICHLORETHYLENE VAPOR; AND THEN MECHANICALLY CLEANED BY BLASTING WITH AN ABRASIVE-WATER SLURRY; THE PARTS ARE THEN WATER RINSED AND FIXTURED IN CONTACT WITH THE CATHODE BUS BAR OF A STANDARD ELECTROPLATING TANK, AND IMMERSED IN A SOLUTION OF COMMERCIAL PLATING CHROMIC ACID WHICH IS MAINTAINED AT A TEMPERATURE BETWEEN 125* F. AND 135*F.; THE ANODES OF THE TANK ARE ADJUSTED TO ALWAYS HAVE A RATIO OF TWICE THE SURFACE AREA OF SAID PARTS, AND ELECTRIC CURRENT IS PASSED FOR 12-18 MINUTES THROUGH THE SOLUTION, AT A RATE SUCH THAT THE CURRENTDENSITY-TEMPERATURE RATIO RANGES FROM 1 AMPERE/INCH2 PER EACH 45*F. TO 1 AMPERE/INCH2 PER EACH 60*F., RESULTING IN A CHROMIUM ELECTRODEPOSIT OF .00015" TO .00025" THICK; THE PARTS ARE THEN BAKED FOR 3 HOURS AT 375*F. 